Optical fiber connectors are used either to connect an optical fiber with another optical fiber or to connect an optical fiber with a positive optical device e.g. light-emitting device or photo-detecting device.
In this invention, optical fibers signify such fibers through which infrared lights e.g. the light of a CO.sub.2 laser can propagate with low loss.
Thus the term optical fiber in this invention does not mean the conventional fibers e.g. fused glass fibers which enable us to transmit visible light (wavelength 0.4-0.75 .mu.m) or near infrared light (wavelength 0.75-2.5 .mu.m).
These conventional optical fibers are generally used to transmit optical, digital or analog signals. The optical fibers are long, light and fine. Typically these optical fibers are a hundred meters to tens of kilometers. The cladding diameter is typically 0.1 mm. They have a core-clad structure. The core is a central portion in which a light beam transmits. The clad is a peripheral portion which encloses the core portion. The refractive index of the core is slightly larger than that of the clad. Although both the core and the clad are transparent to the transmitting light, the light transmits mainly through the core because of the difference in the refractive indices.
However, the optical fibers in this invention transmit the power of infrared light instead of signals. Specifically, the optical fibers can be used to transmit the light of CO.sub.2 lasers or radiation thermometers. A radiation thermometer detects temperature of an object by measuring the power of infrared light emitted by the object. The power spectrum of the emitted lights depend on the temperature of the object.
CO.sub.2 lasers can emit strong infrared light beams with high efficiency. The wavelength of the CO.sub.2 laser light is 10.6 .mu.m in a usual case with a very large light power.
The fused silica fiber or the plastic fiber cannot transmit CO.sub.2 laser light. Various kinds of optical fibers for CO.sub.2 laser light transmission have been proposed. Some of them are glassy fibers. Another are crystalline fibers. Glass fibers are made of chalcogenide glass e.g. As-S glass or As-Se glass, or monoelement glass e.g. Ge glass. However, proposed glassy fibers for CO.sub.2 laser light are generally very difficult to manufacture.
More prospective fibers are crystalline fibers. The following crystalline fibers are known now:
(1) Thallium Halides TlI, TlBR, TlC1 and mixture crystals of them PA1 (2) Silver Halides AgI, AgBr, AgCl and mixtures of them PA1 (3) Alkali Halides CsI, CsBr, etc.
The optical fibers for CO.sub.2 laser light transmission are generally short, and thick. The length is typically 1 m to 3 m and the diameter is typically 0.6 mm to 1.0 mm. Most of the optical fibers have no core-clad structure. They lack a clad portion in general.
The characteristics required for optical fiber connectors are a possibility of coupling and decoupling, a small connection loss, and easy handling.
The connection loss mainly originates from incorrect locations of optical fibers in optical fiber connectors. Due to the incorrect locations when two optical fiber connectors are coupled, the centers of optical fibers become discrepant with each other. Because the diameter of the fiber core is small, even a slight discrepancy in the fiber centers causes a great connection loss.
FIG. 4 shows a sectional view of a conventional optical fiber connector for CO.sub.2 laser light.
At a front end of an optical fiber (21), a cylindrical sleeve (22) is fitted coaxially. The outer surface of the sleeve (22) has been fishished to a cylindrical surface with high accuracy. The sleeve (22) is inserted and fixed in a receptacle (23) having an inner cylindrical surface finished with high accuracy.
The longer the contact length is, the more accurately the center of the sleeve (22) coincides with the center of the receptacle (23).
The narrower the clearance between the outer cylindrical surface of the sleeve (22) and the inner cylindrical surface of the receptacle (23) is, the smaller the vibration of the center of the sleeve (22) in the direction vertical to the axial direction becomes.
However due to the restriction of accuracy of finishing, the clearance between the outer cylindrical surface and the inner cylindrical surface cannot be extinguished completely. Furthermore probable incorrectness of cylindrical surfaces of the receptacle and the sleeve forbids the clearance therebetween from extinguishing completely.
These size errors cause a large energy loss of light at the connecting portion. The large energy loss is a problem for conventional optical fiber connections.
In addition, the energy loss is not a constant value but randomly-dispersed values.
The energy of light transmitting in optical fibers is dissipated at the connecting portion. Because the energy loss is not constant, the connectors cannot be a part of accurate measuring devices, for example a radiation thermometer.
The energy loss at the connection portion causes a serious difficulty in the high power transmission of light. The power of the light emitted from a strong CO.sub.2 laser is very strong. Thus the energy loss at the connecting portion is also strong. The heat generated by the energy loss of laser light may burn and impair the ends of the fibers at the connecting portion.
When there are some size errors in the cylindrical surfaces, some clearances must exist between the sleeve and the receptacle. If not, the sleeve cannot be inserted into the receptacle. Thus some clearance should be required between the sleeve and the receptacle.
However the existence of the clearance makes the location of the sleeve difficult, because the sleeve displaces in radial directions in the receptacle.
When an external force acts upon a portion of the connector or the optical fiber, the sleeve displaces in radial direction owing to the external force. The radial displacement of the sleeve changes the relative positions of the fiber centers which are facing together in the connector. Thus the radial displacements change the connecting loss.
This fluctuation of the connecting loss induces a serious difficulty, because it is caused in a same connector by external forces.
The difficulty of radial displacement of sleeve owing to the clearances cannot be solved so long as we employ the cylindrical surface type of optical fiber connectors.
Then it may be more prospective to adopt conical surface fitting for optical fiber connectors. Here the conical surface fitting means the assembly of the receptacle with an inner conical surface and the sleeve with an outer conical surface.
So long as the top angles of the conical surfaces are common, the sleeve will fit well in the receptacle. If the sleeve is pushed strongly against the receptacle, no radial displacement of the sleeve will occur.
However the cutting of conical surfaces is more difficult than the cutting of cylindrical ones. The size errors of the cutting of conical surfaces are considerably large.
Along a center line of the cone, a small hole through which an optical fiber passes must be perforated. However this perforation is more difficult than the perforation of a hole along a center line of a cylinder. Thus the error of the center of a hole perforated in a cone is large. Furthermore all longitudinal surface lines cannot be correct straight lines. They have concave portions and convex portions more or less.
Therefore if a sleeve and a receptacle were conical, random radial displacement would be forebidden, but the centers of them might deviate in radial directions or the center lines of them might deviate from the parallel relation.
Conical sleeves and receptacles are now only imaginary matters. So long as the Inventors know no conical sleeve nor conical receptacle for optical fibers which transmit infrared light have been manufactured till now.
Thus conventional cylindrical sleeve and receptacles as well as imaginary conical sleeves and receptacles have inherent drawbacks.